Supply voltage is generally very low in energy harvesting applications. As such, a self-starter circuit is required to trigger a semiconductor chip which includes power management circuit to start operate in such a low supply voltage. A ring oscillator is typically used as the self-starter circuit. However, a conventional ring oscillator requires relatively high self-oscillating voltage which is undesirable.
From the foregoing discussion, it is desirable to provide a ring oscillator circuit which can operate at ultra-low supply voltages, such as 45-50 mV or below.